Various techniques for transmitting real-time data to be transmitted in real time and non-real-time data not to be transmitted in real time via the same network are available. For example, a bandwidth control device disposed between a local area network (LAN) and a wireless network detects a bias toward non-real-time communication and performs control to suppress non-real-time communication, thereby reserving fixed bandwidth for real-time communication to reduce overflow in the real-time communication. This technique can ensure the quality of real-time communication that uses fixed bandwidth, such as audio communication. Although this technique may on average ensure the quality of real-time communication such as audio communication, a large delay may sometimes occur.
In another technique, when receiving non-real-time communication data having a preset frame length or longer, a relay device or any other suitable device divides the non-real-time communication data into frames of a length less than or equal to the preset frame length, thereby reducing the occurrence of collisions between real-time communication and non-real-time communication. In this case, collisions may not be completely avoided, and an increase in the number of relay devices along a path would increase the delay. In addition, the quality of real-time communication can be adjusted with the division size of non-real-time communication data; however, the following problems occur. If the division size is too large, the number of collisions increases. If the division size is reduced, the actual bandwidth in a network is reduced because of an increase in header length, and the processing load on a receiving node is increased.
For example, if an output port of a relay device such as a switch from which real-time data is to be output is already being used to output non-real-time data, even though it is possible to preferentially process real-time data, transmission of real-time data may be disturbed by a frame of non-real-time data currently being transmitted at a time when the relay device is receiving the real-time data, and the transmission of the real-time data may be delayed by an amount corresponding to the size of the frame. For example, a 1 Gbps (=125 M Bytes/sec) switch with a maximum transmission unit (MTU) of 9000 bytes has a maximum delay of approximately 72 (=9000/125 M) μs. Since each relay device has a maximum delay as given above, the larger the number of relay devices before a destination node, the larger the delay caused by such frame collision.
Examples of the related art are disclosed in Japanese Laid-open Patent Publication No. 2004-208124, Japanese Laid-open Patent Publication No. 2002-33766, Japanese Laid-open Patent Publication No. 2003-338837, and Japanese Laid-open Patent Publication No. 2007-28638.